1. Field of the Invention
The present invention pertains to suture and surgical prosthesis materials. In particular, it pertains to suture materials and fabrics suitable for production of vascular prosthesis.
2. Description of the Prior Art
Silk has been utilized for many years as a suture material. This material, sometimes called thread, braid, fiber, etc., is usually comprised of continuous strands of woven, braided or twisted discrete silk fibers or filaments of different fineness or denier. The number of silk filaments woven into each thread determines the size (diameter) and strength of the resultant thread. The silk filaments are of a fine continuous protein fiber produced by a variety of insect larvae. Most commonly, the material is a lustrous tough elastic fiber produced by silk worms and used for textile purposes.
The silk filaments are primarily composed of a protein known as silk fibroin. This protein comprises peptide chains crosslinked by hydrogen bonds and disulfide bonds, diester phosphate linkages and/or salt linkages. Thus, the silk molecule is composed of arrays of a nearly homogeneous protein that are arranged in a linear configuration, tenaciously crosslinked with hydrogen bonds between NH and CO groups to form a filament of sufficient strength for use as a thread component.
The silk filament can be fashioned into thread by twisting and the thread may in turn be fashioned into a fabric or cloth by weaving, felting or knitting. Such fabric has the potential to be fashioned into vascular prosthesis for use in animal recipients to replace occluded, damaged and/or diseased arteries and/or veins. The fabric may be fashioned into a variety of shapes by cutting and sewing using silk threads to join the pieces together. Thus, the formed device or prosthesis may be totally made of silk.
The protein of the silk thread, when used as a suture material or in a prosthesis, evokes or elicits an antibody reaction in the host tissue within a matter of weeks. The antibodies are probably produced and are directed against particular sites on the silk protein. The silk protein is a foreign protein in the host, thus becoming an effective antigen. Thus, silk thread which enjoys most of the physical qualities making it an ideal suture (easy to knot, flexibility, ease of handling, pliant, lacking stiff projecting ends when cut, easy to remove, etc.) suffers from its proteinaceous nature. It induces a host related cellular reaction. This cellular reaction is in all likelihood a true inflammatory reaction initially and evokes the migration of polymorphonuclear cells, lymphocytes and macrophages to the silk-tissue interface. The enzymatic reduction of the silk molecule is probably mediated by these cells. This may later pass into a true foreign body reaction with the macrophages attracting fibroblasts. Fibroblasts produce a collagenous capsule around the thread or prosthesis structure.
Enzymes produced by the macrophages slowly digest the silk in situ. After a period of one year in tissue, the silk material will have lost its tensile strength and in approximately two years will have disappeared completely.
In the case of skin sutures, the epidermis around the suture entry or exit points migrates down around the thread (suture tract). Thus, the suture itself becomes an irritant and the site of skin penetration becomes a wound. The removal of the silk suture ultimately leaves a scar. This is because the migrating epithelium produced at the wound margin migrates over the suture wound, meets and creates a small pit or dimple at each site. This can be prevented somewhat by removing the sutures early (sometimes before adequate wound strength has been attained).
If silk fabric is used as a vascular prosthesis material, its slowly absorbable nature will render the prosthesis unusable after a period of time, requiring its total replacement.
Thus, while silk, from the standpoint of flexibility, handling, etc. is ideal for sutures and a prosthesis material, its antigenicity and slowly absorbable nature, render it unsuitable for many applications. For this reason, other materials, lacking the flexibility, ease of handling, etc. of silk, are used for sutures and in the construction of vascular prosthesis.